While catalytic converters are well known for reducing oxides of nitrogen (NOx), and oxidizing hydrocarbons and carbon monoxide from automobile exhaust, these reactions typically take place after the catalyst has attained its light-off temperature, at which point the catalyst begins to convert the hydrocarbons to harmless gases. The typical catalytic light-off time for most internal combustion engine systems is around 50 to 120 seconds (generally in the temperature range of 200.degree.-350.degree. C.); with the actual catalytic light-off time for any system depending on a number of factors, including, the position of the catalyst relative to the engine, the aging of the catalyst, as well as the noble metal loading. Seventy to eighty percent of hydrocarbon emissions from automotive vehicles are emitted during this first minute, or so, of "cold start" engine operation, i.e., without additional measures large amounts of hydrocarbons are likely to be discharged into the atmosphere during this period. The problem is made worse by the fact that the engines require rich fuel-air ratios to operate during cold-start thus, increasing even further the amount of unburned hydrocarbons discharged. Environmentally, increasing the effectiveness of automotive emission control systems during cold start, so that the amount of hydrocarbons discharged into the atmosphere during cold-start are kept at extremely low levels, has become increasingly important.
Various schemes have been proposed for meeting the stringent hydrocarbon emission standards during cold start including, the use of electrically heated catalysts (EHCs) to reduce the light-off time of the main catalyst, the use of molecular sieve structures (hydrocarbon adsorbers) to adsorb and hold significant amounts of hydrocarbons until the converter has attained its light-off temperature, as well as combinations of both.
Recently, improved in-line and by-pass exhaust control systems respectively have been disclosed in, co-assigned U.S. applications Ser. No. 08/375,699 (Guile et al.) and U.S. Ser. No. 08/484,617 (Heft et al.); both herein incorporated by reference. The Guile reference discloses a by-pass adsorber system wherein flow patterns from a secondary air source are used to direct exhaust gas flow to and away from the adsorber during cold-start.
Hertl discloses an in-line exhaust system having a main catalyst, a housing downstream of the main catalyst having an inlet and an outlet end, and having disposed therein a molecular sieve structure for adsorbing hydrocarbons. The molecular sieve structure exhibits (1) a first region forming an unobstructed or substantially unobstructed flow path for exhaust gases of an exhaust stream; and, (2) a second, more restricted flow path, region adjacent the first region. Furthermore, Hertl's exhaust system includes a burn-off catalyst disposed downstream from the adsorber having a light-off temperature. Lastly, the system includes a diverting means disposed in the housing for passing secondary air to the molecular sieve structure; the flow pattern of the secondary air directs a major portion of the exhaust gases of the exhaust stream through the second region of the adsorber prior to the main catalyst attaining its light-off temperature.
Environmental concerns and legislation drafted to meet those concerns continues to lower legally acceptable hydrocarbon emission standards, e.g., the California ultra-low emission vehicle (ULEV) standards. Notwithstanding the fact that the Hertl system outperformed earlier exhaust systems, there thus continues to be a need for improvements to existing systems and for new systems capable of meeting these stricter exhaust emission standards.